Author:

Mikhail Anisimov(University of Maryland, College Park)

Twenty years ago it was suggested that the anomalous properties of
supercooled water may be caused by a critical point that terminates a line
of metastable liquid-liquid separation of lower-density and higher-density
water. I describe a phenomenological model in which liquid water at low
temperatures is viewed as an athermal solution of two hydrogen-bond network
structures with different entropies and densities. Alternatively to the
lattice-gas/regular solution model, in which fluid phase separation is
driven by energy, the phase transition in the athermal two-state water is
driven by entropy upon increasing the pressure, while the critical
temperature is defined by the reaction equilibrium constant. The order
parameter is associated with the entropy, while the ordering field is a
combination of temperature and pressure. The model predicts the location of
density maxima at the locus of a near-constant fraction of the lower-density
structure. Another example of entropy-driven liquid polyamorphism is the
transition between the structurally ordered ``Blue Phase III'' and
disordered liquid in some chiral materials; this transition is
experimentally accessible. I also discuss the application of the two-state
model to binary solutions of supercooled water in which liquid-liquid
transition may also become accessible to direct observation. Some atomistic
``water-like'' models such as mW, do not show liquid-liquid separation in
the metastable liquid domain. However, even without actual liquid-liquid
separation, the anomalies observed in MD simulations of mW can be accurately
described by the entropy-driven nonideality of two molecular configurations,
the same physics that presumably drives the liquid-liquid transition in real
water.

*This research is supported by NSF grant no. CHE-1012052.

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2013.MAR.N42.2